| Abstract|| |
Background: For porcelain fused to metal restorations, computer-aided designing (CAD) / computer-aided manufacturing (CAM) systems claim to provide improved marginal fit than conventional casting systems. Aim: The present in-vitro study was conducted to compare the marginal fit of cobalt-chromium alloy copings fabricated with milled wax/lost wax, CAD/CAM milling/sintering and direct metal laser sintering (DMLS) techniques. Materials and Method: A metal die simulating a prepared tooth was fabricated and scanned using an optical scanner. A standardized coping design was used to manufacture 30 alloy copings divided into three groups of ten copings each i.e., milled wax/lost wax, milling/sintering and DMLS. A modified replica technique was used to measure the silicone film thickness at four pre-designated points on the margin under a digital stereo-microscope. Statistical Analysis: The mean values for marginal gap were compared using one-way Analysis of Variance (ANOVA) followed by Tukey-Kramer test for all pair-wise comparisons. Results: The mean marginal gap values obtained by using milled wax/lost wax technique, milling/sintering technique and DMLS technique were 88.44 μm, 61.135 μm and 55.39 μm, respectively. One-way ANOVA showed a significant difference for marginal fit between the lost wax group and the other two test groups (P < 0.05). Conclusion: The cobalt-chromium alloy copings fabricated using DMLS techniques displayed significantly better marginal fit than the milled wax/lost wax technique which may contribute to their improved clinical performance. DMLS technique produces restorations with an improved marginal fit and may be preferred by clinicians over milling and conventional casting techniques.
Keywords: Computer aided design, dental casting technique, marginal fit, porcelain-metal alloys
|How to cite this article:|
Sarda AS, Bedia SV. Influence of manufacturing technique on marginal fit of cobalt chromium restorations: An in-vitro Study. Indian J Dent Res 2021;32:495-9
|How to cite this URL:|
Sarda AS, Bedia SV. Influence of manufacturing technique on marginal fit of cobalt chromium restorations: An in-vitro Study. Indian J Dent Res [serial online] 2021 [cited 2023 Feb 3];32:495-9. Available from: https://www.ijdr.in/text.asp?2021/32/4/495/345414
| Introduction|| |
Many tooth-coloured options are available for the fabrication of posterior fixed partial denture prostheses wherein porcelain-fused-to-metal (PFM) prostheses are still the most commonly used in clinical practice. For fabricating PFM prosthesis, the use of base metal alloy copings such as cobalt-chromium (Co-Cr) is preferred as it reduces costs compared to noble metals and provides satisfactory clinical performance. Electrochemical studies show that Co-Cr alloys are also more resistant to corrosion than nickel-chromium (Ni-Cr) alloys. Nickel-based alloys have a greater sensitization potential than cobalt-chromium alloys, whereas Co-Cr alloy allergies are rare. Furthermore, the casting of Co-Cr alloys continues to be a routine procedure in many dental laboratories.
Precise marginal fit is necessary to achieve a better mechanical, biological and aesthetic prognosis of cemented restorations. Studies in the literature have revealed that gingival tissue adjacent to the margin of an artificial crown contained chronic inflammatory infiltrate which is believed to have occurred due to the accumulation of bacterial plaque at the microscopic opening of margins of the restoration. Increased marginal gap is responsible for plaque retention, microleakage and cement breakdown which may lead to endodontic and periodontal complications. In addition, the longevity and strength of restoration with poor marginal fit may be reduced due to stress concentrations created by variations in the fitting. The maximum clinically acceptable marginal gap has been reported to be 120 μm. The marginal gap of indirect restorations depends upon the accuracy of clinical procedures performed by the clinician and the type of manufacturing technique used by the laboratory.
The conventional lost wax technique offers advantages of convenient laboratory handling. However, it is time-consuming and technique sensitive and has several drawbacks related to wax thermal sensitivity, high coefficient of thermal expansion, distortion during removal from die, shrinkage of molten metal all of which compromise the marginal fit and clinical longevity of such restorations.
Newer computer-aided designing (CAD) computer-aided manufacturing systems (CAM) techniques have been introduced such as subtractive milling/sintering and additive direct metal laser sintering (DMLS) which claim to provide precise restorations with minimal marginal gap and may replace the lost wax technique. Very little data exists, which can guide the clinician to select a manufacturing technique to achieve an optimum fit and decrease the risk of failures. Studies comparing the marginal fit of alloy copings fabricated using conventional and newer CAD/CAM techniques show inconclusive results.,
Therefore, the present in vitro study was conducted to evaluate and compare the marginal fit of Co-Cr alloy copings fabricated using milled wax/lost wax, CAD/CAM milling/sintering and DMLS techniques.
| Materials and Method|| |
The present in-vitro study was conducted in the Department of Prosthodontics and Crown & Bridge of a dental institution. The ethical committee approval was obtained on 12-04-2018.
Fabrication of custom-made stainless-steel die
A milled stainless-steel die simulating a prepared tooth was fabricated comprising of the following sections [Figure 1]:
- Tooth preparation section
- Cylindrical section with grooves
- Base for mounting
A 1 mm deep chamfer finish line was provided, and the height of the preparation was 8.015 mm. A single 45° occluso-buccal bevel of 2 mm length and 0.5 mm depth similar to a functional cusp bevel was made. Four shallow grooves were made around the cylindrical section. One buccal groove on the cylindrical section coincided with the midpoint of the occluso-buccal bevel to aid in the correct orientation of copings. Small indentations at four different points (mid buccal, mid lingual, mid mesial, mid distal) were made 1 mm below the margin on the cylindrical section. The base of the die was mounted in autopolymerising acrylic resin, and the occlusal half of the preparation area was sandblasted.
Fabrication of alloy copings
The metal die was scanned using an optical scanner (DS-EX Shining 3D, China). During the CAD process, an elevated ridge was added on the cameo surface of coping, exactly overlying the buccal groove for predictable seating of copings. The overall coping thickness was set at 1 mm, and the cement space was set at 70 μm including the marginal area. Thirty copings were fabricated by three different manufacturing techniques i.e., milled wax/lost wax (Group A), CAD/CAM milling/sintering (Group B) and DMLS (Group C) with ten copings in each group.
Fabrication of milled wax/lost wax copings
The CAD data were transferred to the milling machine (Arum Dental Products, China), and ten wax patterns were produced through a subtractive milling method [Figure 2]a. The wax patterns were connected to a manifold sprue and invested using graphite-free, phosphate- bonded investment material (Bellosun, Bego, Germany), and a burnout procedure was carried out. The casting was accomplished using a Co-Cr alloy (Wirobond-C, Bego, USA) melted in an induction casting machine (Fornax Geu, Germany) as per manufacturers' protocols. The internal surface of the copings was inspected under magnification for any irregularities followed by steam cleaning. All the ten copings were found to be acceptable and included in the study [Figure 2]b.
|Figure 2: (a) Milled wax copings (b) Lost wax metal copings (c) Milled/Sintered metal copings (d) Laser sintered metal copings|
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Fabrication of CAD/CAM milling/sintering copings
Data from CAD software were transferred to a milling machine where soft Co-Cr blank was processed to a pre-state material by dry milling process. The milled copings were sintered to full density in a special, high-temperature sintering furnace under an argon protective gas atmosphere at 1300°C. All ten copings were included in the study after thorough inspection [Figure 2]c.
Fabrication of DMLS copings
The CAD data were fed into the building chamber of a laser sintering machine (EOS Cobalt Chrome SP2, USA) where the infrared laser beam fused the Co-Cr alloy powder, layer by layer to produce ten alloy copings. All ten copings were included in the study after thorough inspection [Figure 2]d.
All the 30 copings were labeled and checked under magnification for complete seating. A thin layer of petroleum jelly was applied inside the copings except near the margins. Thirty autopolymerizing resin trays with 4 mm uniform spacing were prepared over the die, and their borders were trimmed 3 mm short of the margin. Tray adhesive was applied and kept aside to dry.
Modified silicone replica technique
The intaglio surface of coping was coated with sufficient silicone indicator material (Fit Checker Advanced, GC, U.S.A.) and seated slowly on the die using the elevated ridge and buccal groove as a guide. Care was taken to allow the excess silicone to be extruded gradually at the margins by using a mild wiggling motion until the coping was seated completely. A constant vertical load of 50 N was exerted on the coping using a Universal Testing Machine (Unitest 10, Acme Engineers, India) at a cross-head speed of 1 mm/min for 5 minu. Once the silicone was set completely, the coping was removed such that the silicone film remained attached to the roughened die. The thin silicone film was now picked up using contrast colour putty in the prepared custom tray. The film along with supporting putty was cross-sectioned vertically at four points corresponding to the replica of indentations made on the die and film thickness measured at these points.
Evaluation of marginal fit
The cross-sectional thickness of silicone film was measured on images obtained at 20 × magnification using a digital stereo-microscope (Wuzhou, New Found Instrument Co. Ltd., China) [Figure 3]. The marginal gap was measured by employing an image measuring system (MVIG 2005, Chroma Systems Pvt. Ltd., India). Images of the samples were projected on the computer screen, and measurements were made using the in-built software in microns. The data obtained were tabulated, and the mean was derived for each point of measurement after subtracting 70 μm (default cement space) for all the three groups.
|Figure 3: Cross-sectional thickness of silicone film measured using a digital stereomicroscope (magnification 20 ×)|
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All statistical measurements were carried out using Medcalc version 18 software. The mean values for marginal fit were compared using one-way analysis of variance (ANOVA) followed by Tukey-Kramer test for all pair-wise comparisons.
| Observations and Results|| |
The mean marginal gap represented by the mean thickness of silicone film at all four points for copings in Group A, Group B and Group C was 88.44 μm, 61.135 μm and 55.39 μm, respectively [Table 1]. The results of ANOVA indicated a significant difference between Group A and the other two groups (P < 0.05). There was no significant difference between Group B and Group C, which exhibited close mean values for marginal gap [Table 2]. The CAD/CAM milling/sintering and DMLS copings exhibited lesser marginal gap values at all the four measured points as compared to the lost wax group.
|Table 1: Mean marginal gap values in microns (μm) at four predetermined reference points|
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|Table 2: Comparison of the marginal fit between all three-group using Tukey-Kramer test for all pairwise comparisons with standard deviation and P value|
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| Discussion|| |
Ideally, cemented crown margins should meet prepared tooth margins in perfect non-detectable junctions. However, clinically such perfection may be difficult to achieve and equally difficult to verify. McClean and von Fraunhofer proposed a minimal marginal gap of less than 120 μm may be considered clinically acceptable. The marginal discrepancy of 25 μm for cemented restorations has been suggested as an ideal clinical goal as per the American Dental Association Specification no. 8 for maximum film thickness; however, marginal openings of this size are seldom reported. In this study, all the copings exhibited a clinically acceptable range (57.68 μm–98.8 μm) of marginal fit irrespective of the manufacturing technique used. However, DMLS copings showed the least marginal gap values followed by the milling/sintering technique.
A relatively higher mean marginal gap of 88.44 μm for the lost wax group may be attributed to the casting shrinkage of molten alloy. The induction heating coil used during casting may melt alloy at a higher temperature than its melting range which may cause the alloy to lose its low melting point compositional elements making it more viscous and affecting its flow. Another factor may be the delayed time taken to melt alloy in an electrical machine, a condition that may also modify the alloy's composition and consequent viscosity. Based on the results obtained, it may be stated that CAD/CAM techniques aid in enhancing the precision of restorations by overcoming these casting-related errors.
Ullattuthodi S et al. and Hong MH et al. concluded that DMLS technique had issues of binding faults (lack of fusion), probably because of incomplete melting of the powders due to insufficient energy density which may cause distortion. However, in this study, DMLS technique yielded copings with better marginal fit than those made using the milling technique. This may be attributed to differences in the operating software and the machines of different manufacturers.
During the CAD process, the internal cement space is provided as a default to substitute for the die spacer which is used on stone dies to allow space for the luting cement and allow complete seating of the crowns. The default space for copings of all three groups was set uniformly at 70 μm to eliminate the possibility of seating interferences. This 70 μm was subtracted from the observed value of the marginal gap and noted at the time of recording the measurements. The mean value thus obtained was used for intergroup comparison, so that influence of cement space on the results could be minimized.
Huang et al. used an impression replica technique in their study for marginal gap measurements. This technique has some disadvantages such as difficulty in selecting the points of measurements, difficulty in locating the most apical portion of the preparation margin, tearing of silicone film upon removal and errors in sectioning plane which could lead to overestimated measurements. The modified impression replica technique used in the present study helped in overcoming these limitations. The metal die was sandblasted before cementation which allowed the silicone film to remain attached to the die surface. The silicone film and putty used were of contrast colour to facilitate quick demarcation on the microscope image. Four external surface indentations were made on the die beyond the chamfer margin which were replicated as tiny elevations in silicone film which served as reference points for sectioning the film.
Groten et al. reported that measurements at approximately 50 points on a single coping were needed for clinically relevant information about the gap size regardless of the gap definition or cementation condition. In this study, only four points were used to measure the marginal gap which may be a limitation of the study. A cylindrical metal die geometry would result in non-uniform placement of copings. Hence, features such as an occlusal bevel, vertical grooves on die and an elevated ridge on the cameo surface of coping were incorporated to ensured uniform seating of copings by providing a visual guide and rotational resistance. Although utmost care was exercised during the silicone replica procedure, few copings may have been seated in a slightly different position, but this would not have a bearing on the overall results as they were based on the mean of gaps at four widely spaced points.
In the present study, it was observed that the manufacturing technique had a significant impact on the marginal fit of Co-Cr alloy copings. Similar marginal fit accuracies have been demonstrated by Lövgren N et al., Tara et al., Gunsoy S, Bhaskaran E, Harish V and Xu D who concluded that laser-sintered Co-Cr copings showed better marginal fit than copings produced by milling/sintering or milled wax/lost wax techniques.
For fabricating copings by lost wax technique, milled wax copings were used. This eliminated the errors which may get incorporated in the casted coping shapes due to wax-related distortion during handling. More significantly the standardized coping design permitted a reliable comparison of manufacturing techniques as the wax copings were milled from the same CAD file which was used in the two techniques.
| Conclusion|| |
The marginal fit of Co-Cr copings fabricated by all three techniques was clinically acceptable. There was a statistically significant difference in the marginal gap of Co-Cr copings, obtained by milled wax/lost wax technique and the two CAD/CAM techniques. Co-Cr copings fabricated by DMLS had the least marginal gap followed by milled/sintered and milled wax/lost wax technique, respectively. Further studies using porcelain veneered copings and different finish line configurations employing a more sophisticated gap measurement system are needed to reliably conclude the clinical acceptability of marginal fit provided by the newer CAD/CAM techniques.
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Conflicts of interest
There are no conflicts of interest.
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Dr. Abhishek S Sarda
Sardawada, Mangrulpir, Dist- Washim - 444 403, Maharashtra
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]